Thermoset-thermoplastic molded article for dental restoration

ABSTRACT

For use in making dental restorations, a model is prepared by taking an intra-oral impression and molding in the impression a replica model made from the thermosetting resin and a thermoplastic resin. The thermosetting resin provides a tough, heat- and abrasion-resistant surface. Heat from the molten thermoplastic resin accelerates the curing of the thermosetting resin.

This is a continuation of application Ser. No. 932,005, filed Nov. 18,1986, now U.S. Pat. No. 4,850,871.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention primarily concerns restorative dentistry, especiallymethods and devices useful for making out-of-the-mouth or extra-oraldental restorations from dental restorative or composite resinmaterials. The invention also concerns a model or die from which aprosthetic dental restoration can be made and a method of making such amodel or die. The invention further concerns models or dies useful forsuch purposes as the restoration of art objects such as statues.

2. Description of Related Art

In spite of remarkable technological advances in prosthetic dentalrestorative or composite resin materials, amalgams typically are easierto install, can be completed in a single visit, and are regarded by manypractitioners as having superior durability. For such reasons, theamalgams continue to predominate in posterior dental restorations inspite of their toxicity, aesthetically undesirable color, and the usualneed to remove healthy portions of a tooth in order to interlock theamalgam into a cavity. Dental restorative or composite resins also canbe applied in a single visit by being tamped into a cavity, shaped orsculptured, then cured by exposure to light and finished with a bur. Thestep of shaping or sculpturing before curing is cumbersome, as isgrinding after curing. Furthermore, shrinkage of the resin during curingproduces strain on the tooth and can result in marginal leakage. Evenwhen shrinkage is minimized by incremental curing and the dentist hassufficient skill to sculpture the uncured resin to duplicate theoriginal tooth contour precisely, the procedure is sufficientlydemanding and time consuming that the dentist may prefer the convenienceof an amalgam.

It has been suggested that the effect of resin shrinkage can beminimized by using a model or die to form an extra-oral prostheticdental restoration such as an inlay. Such a model can be formed fromdental or gypsum stone (Plaster of Paris), from thermoplastic resin asin U.S. Pat. No. 2,135,404 (Wheeler), or from epoxy resin as illustratedin Jensen et al., "Polymerization Shrinkage and Microleakage," a paperpublished in Posterior Composite Resin Dental Restorative Materialsedited by Vanherle et al., pages 243-262 (Peter Szulc Publishing Co.,The Netherlands, 1985). The Jensen article in a table at page 258 listsadvantages and disadvantages of each of in-the-mouth andout-of-the-mouth "`inlay` posterior composites," the advantages of thelatter being:

"Reduced stress on cusps from polymerization shrinkage

Better marginal adaptation at gingivo-proximal (no overhang)

Control of proximal contacts

Better contours and anatomy

Easier to obtain a better surface finish

Possible increased abrasion resistance because resin can be heat curedunder vacuum"

Among the listed disadvantages of the out-of-the-mouth or extra-oral"`inlay` posterior composite" are that normally more than one dentalappointment is required, thus requiring a temporary restoration, andthat there is increased cost due to laboratory procedures.

While the Jensen article refers to the use of an epoxy die for moldingdental restorations, and the Wheeler patent refers to the use of dentalpatterns made from certain thermoplastic resins, such dies or patternsare more commonly made from dental or gypsum stone. Gypsum stone isgenerally regarded as the state of the art molding material againstwhich other materials are measured. The thermoplastic resins of theWheeler patent are said to be grindable but must be melted at fairlyhigh temperatures. This can cause unacceptable shrinkage of the modeland poor restoration fit. Like epoxy resin, gypsum stone takes a longtime to harden, thus requiring two visits to the dentist and a temporaryrestoration between visits. The need for two visits can be exceedinglyinconvenient to patients who live in remote areas, and the need for adentist to use a dental laboratory can be troublesome when the closestlaboratory is at a distant location.

SUMMARY OF THE INVENTION

The present invention permits an extra-oral prosthetic dentalrestoration to be made in a single visit. It can enable attainment ofthe advantages quoted above from the Jensen article, while eliminatingor minimizing the above-mentioned disadvantages. The invention can alsobe used for nondental restoration work to provide a durable model thatcan be used within minutes after it is made. These advantages areachieved by a method comprising the steps of:

(1) forming a rubbery, heat-resistant impression of an object to beduplicated (e.g., a tooth, teeth, gingival or gum tissue, or otheranimate or inanimate object),

(2) partially filling the impression, e.g., coating or dusting all or apart of the working (e.g., tooth) surfaces of the impression, with aliquid or powdered thermosetting resin,

(3) further filling said impression with a molten thermoplastic resinand, after the thermoplastic resin solidifies and the thermosettingresin is thermoset,

(4) removing the thermoset resin and solidified thermoplastic resin fromsaid impression to provide a model of said object.

The thermoset surfaces of the model can be machined more readily than amodel made entirely from thermoplastic resin. This is important when themodel is of human teeth, because it often is necessary to grind offmaterial, e.g., at the gingival margins. A surface of cured thermosetresin is also useful when the model is to be used to shape a dental ornondental restoration, because the thermoset resin typically willexhibit good wear resistance. This is important when the restoration isrepeatedly installed on and removed from the model and especiallyimportant when the restoration comprises a metal such as gold. Becausethe thermoplastic portion of the model need not be made excessivelyheat- or abrasion-resistant, thermoplastic resin with optimal meltingtemperatures and shrinkage can be employed. In addition, the cure rateof the thermosetting resin is greatly enhanced when the impression isfilled with molten thermoplastic resin in step (3), thus rapidlypolymerizing the thermosetting resin to a tough, abrasion- andheat-resistant state, and enabling the model to be used within minutesafter it is molded.

The model produced by the above 4-step model-making method is itselfbelieved to be novel and has a variety of dental and nondental uses. Forexample, the foregoing steps can be followed by the steps of:

(5) applying restorative resin to a portion of the restoration isrequired (usually after first applying a release agent to the model),

(6) shaping or sculpturing the applied restorative model where resin toa desired contour, and

(7) curing the restorative resin to provide a restoration.

When the restoration is a dental restoration and is cemented into thepatient's mouth, the cement can compensate for polymerization shrinkageof the material used to make the restoration and thus provide goodassurance against microleakage. The same 7-step method can be used fornondental restorations, for example, to repair art objects such asmarble statues, especially where the restoration should have the sameform and contour as the object being restored.

A primary advantage of the novel method and model in dental use is thatthe model can be made far more quickly than the gypsum or epoxy modelsthat are currently in use. Thus a model of a tooth or teeth can beprepared, used to make a prosthetic dental restoration, and therestoration can be bonded to the tooth or teeth, all in a single visitto the dentist. This eliminates any need for a temporary restoration.While the impression-forming step (1) of the novel model-making methodrequires the same length of time as do methods used for currentextra-oral restorations: steps (2) and (3) of the method provide asubstantial time saving in that a combination of thermoset andthermoplastic resins can harden much faster (e.g. within a few minutes)than can gypsum stone or models made entirely of epoxy resin. Anadditional advantage is that a dental auxiliary (rather than a dentist)can carry out steps (2) through (7). Meanwhile the dentist can work onanother patient, returning to the first patient when it is time toinstall the restoration. This can reduce cost, since the training andskill of a dentist customarily commands a wage far greater than that ofa dental auxiliary. While the same cost reduction is available incurrent extra-oral restorations, that reduction may be more than offsetby the cost of transfering impressions or models between the dentist'soffice and a dental laboratory and scheduling an extra patient visit.

In a preferred embodiment, the novel model-making method can be modifiedby including between steps, (3) and (4) an additional step of adhering aflexible, dimensionally, stable support to the thermoplastic resin.Models made by this modified method can be flexed by hand (or cut with aknife or other sharp instrument) to form in the solidified thermoplastioresin clean cracks at one or more locations (e.g., in the interproximalspaces flanking a replica tooth). With the flexible support serving as ahinge, the model then can be opened at those cracks to isolate andexpose a portion of the model (e.g., the mesial and distal surfaces of areplica tooth). In this fashion the model can be easily manipulated tofacilitate access to a portion of the model, e.g., a single replicatooth.

BRIEF DESCRIPTION OF THE DRAWING

In the drawing:

FIG. 1 is a side elevation of a first model made in accordance with theinvention;

FIG. 2 shows the model of FIG. 1 flexed to expose proximal regions ofone of the replica teeth on which a prosthetic dental restoration can becreated; and

FIG. 3 is a side elevation of a second model of the invention, cut awayto a central section.

DETAILED DESCRIPTION

Referring to FIG. 1, a rubbery poly(vinyl siloxane) dental impressionmaterial (not shown) has been used to mold a model 10 having replicateeth 11a, 11b, 11c and 11d, all made from a thermoset resin, andgingival tissue 12, made from a first thermoplastic resin. Bonded to thebase of the replica gingival tissue 12 is a flexible support 16,preferably a second thermoplastic resin that is tough and flexible. Thereplica teeth are tough and have good heat- and abrasion-resistance.Their rate of polymerization (cure) was accelerated by contact with thefirst thermoplastic resin in its molten state.

Illustrating a preferred embodiment of the invention, two cracks 17 havebeen initiated in the replica gingival tissue 12 at each side of thereplica stump 15. This is readily accomplished by scoring the firstthermoplastic resin with a razor blade, and then flexing the model topropagate the cracks.

The model is shown in FIG. 2 in its flexed position, providing readyaccess to replica stump 15 (shown in phantom). Restoration 21 (a fullcrown) is shown in its installed position on replica stump 15.Interproximal contacts between restoration 21 and adjacent thermosetreplica teeth 11c and 11d can be checked by returning the model to theoriginal unflexed position shown in FIG. 1.

The first thermoplastic resin preferably is cleanly breakable tofacilitate isolation of replica tooth stump 15. By "cleanly breakable"is meant that when a panel of the thermoplastic resin 1.27 cm (1/2 inch)in thickness is scored and flexed by hand at room temperature, it willbreak at the score to form two mating surfaces without visibly apparentelongation.

In FIG. 3, each replica tooth of a model 30 (which was formed from arubbery dental impression, not shown) has a core 32 of a firstthermoplastic resin and a thin shell 33 of a thermoset resin thatprovides good abrasion- and heat-resistance. The thermoset resin can beformed in place by inverting the impression and coating the replicatooth surfaces and the gingival margin portion of the model with uncuredthermoset resin in liquid or powder form. The first thermoplastic resinis then added to the impression in molten form and permitted to harden.Heat from the molten first thermoplastic resin accelerates the cure ofthe thermoset resin. The first thermoplastic resin provides the bulk ofthe replica teeth and replica gingival tissue 34, thus providing a costsavings and rapid hardening. However, the thermoset shell 33 provideswear resistance and facilitates removal by grinding of replica gingivaltissue, to provide good access for trial installation of a restoration.Bonded across the base of the replica gingival tissue 34 opposite to thereplica teeth is a flexible support 36 of tough and flexible secondthermoplastic resin that can act as a hinge at cracks 37 through thereplica gingival tissue, thus permitting the model 30 to function in thesame manner as shown in FIGS. 1 and 2. The flexible support 36 hasserrations 38 and is pressed as a preformed solid strip into the moltenfirst thermoplastic resin forming the replica gingival tissue 34. Theserrations and relatively weak bond between the first and secondthermoplastic resins permit a replica prepared tooth 35 and replicagingival tissue between the cracks 37 to be removed as a unit 39 fromthe model 30, the unit being repeatedly returnable to its exact originalposition.

The impression material from which a mold of the object to be duplicatedis formed, and in which the model is molded, is a rubbery curablematerial having sufficient heat resistance to withstand the heat of themolten thermoplastic resin. Suitable impression materials includeaddition cure or condensation cure silicones, polyethers andpolysulfides. The silicones are preferred, since the polyethers andpolysulfides generally require the use of a release agent to facilitateremoval of the hardened model. Alginates and hydrocolloids are atpresent unsuitable, since they do not have sufficient heat resistance.

The thermosetting resin should be a one-part or multi-part thermallycurable resinous material that is sufficiently tough and sufficientheat-and abrasion-resistant in its cured state to enable the model to bewaxed or subjected to repeated trial fits of a restorative. Best resultshave been achieved when the thermosetting resin has been a two-partepoxy resin, but excellent results have also been achieved when thethermosetting resin has been a two-part urethane resin. Other usefulthermoset resins are described in "Powder Coatings", Kirk-OthmerEncyclopedia of Chemical Technology, 3d. Ed., Vol. 19, pp. 1-27 (1982),and include polyurethanes and polyacrylics. Phenolic resins also can beused. The thermosetting resin may be applied as a liquid or a powder atelevated or room temperature. Preferably it forms a thin shell on one ormore working surfaces of the model, and has a thickness of less thanabout 2 mm. For dental models, the thermosetting resin can form an outershell or the whole of the replica teeth, and/or a portion of the replicagingival tissue if desired.

The first thermoplastic resin is, as noted above, preferably cleanlybreakable. Useful cleanly breakable thermoplastic resins which have gooddimensional stability include aromatic thermoplastic resins such ascopolymers of vinyltoluene and alpha-methylstyrene, polyamides, andpolyesteramides. The ability of a panel of the resin to break cleanlycan be enhanced by adding fillers such as quartz, glass microbubbles,aluminum powder, carbon black, titanium dioxide, or microcrystallinewaxes. Clean breakability also can be enhanced by the addition of glassymodifiers such as rosin, rosin esters, aliphatic hydrocarbon resins,aromatic hydrocarbon resins, polyterpenes and combinations thereof.

The first thermoplastic resin preferably hardens as rapidly as possible,coincident with maintenance of adequate dimensional stability and otherdesired physical properties. Hardening can, if desired, be acceleratedby quenching the model in a suitable cooling medium (e.g., water) whilethe first thermoplastic resin hardens.

The flexible support optionally used between steps (3) and (4) of themethod is, as noted above, preferably a second thermoplastic resin thatis tough and flexible. The support also may be a fabric (woven ornonwoven) which can be impregnated with a resin, a plastic film such aspolypropylene or oriented poly(ethylene terephthalate), an adhesivetape, leather, or rubber. The support can be filled, e.g., withmagnetizable particles to secure the model releasably to a metal sheetsuch as a metal wing of an articulating jig. The preferred tough andflexible second thermoplastic resin can simply be poured to form a layerover the first-mentioned thermoplastic resin, or can, as noted above, beapplied as a preformed solid strip while the first-mentionedthermoplastic resin is molten. The preformed strip may, if desired, beformed with serrations or knobs.

Useful second thermoplastic resins are sufficiently tough and flexibleto permit repeated (e.g., half a dozen times or more) flexing of themodel between the positions shown in FIGS. 1 and 2 without causingapparent distortion of the model. Such resins include ethylene/vinylacetate copolymers, styrene-butadiene block copolymers, butyl elastomersand polyamides, any of which may be compounded with resins,plasticizers, extenders and fillers to provide desired physicalproperties such as flexibility, adhesion, and dimensional stability.

Dental models of the invention may be used to make any prosthetic dentalrestoration including inlays, onlays, veneers, crowns, and bridges.While each of these can be made entirely from dental restorative orcomposite resin, other useful restorative materials such as metals(e.g., gold), ceramics (e.g., porcelain), and metal-ceramic combinationscan be formed on the novel models disclosed above. Pins or otherrepositionable locating means (optionally coated with a suitable releaseagent) can be installed in the model if desired, to facilitate removaland replacement of individual model teeth.

In the following examples of thermoplastic and thermosetting resins andmodels of the invention, all parts are by weight.

Thermosetting Resin A

This two-part epoxy resin is a liquid having a gel time of 4 minutes at21° C. It contains the following ingredients:

Thermoplastic Resin E

Using the same procedure the following ingredients were mixed withheating and stirring:

    ______________________________________                                        Ingredient                Parts                                               ______________________________________                                        "Piccotex" 100            240                                                 Ethylene/vinyl acetate copolymer, 18% vinyl                                                             88                                                  acetate ("Elvax" 410, E. I. du Pont                                           de Nemours)                                                                   Modified rosin, acid No. 94 ("Regalite" 355,                                                            160                                                 Hercules)                                                                     "Imsil" A-25              246.4                                               "Ti-Pure" R-960           40                                                  Red iron oxide            1.6                                                 Hollow glass microbubbles, avg. density                                                                 24                                                  0.23 g/cm.sup.3 (3M)                                                          ______________________________________                                    

Linear shrinkage of a bar of the resulting Thermoplastic Resin E was0.056 cm (0.022 in.) or 0.44%.

EXAMPLE 1

A dental model was made as illustrated in FIG. 3. The teeth werereplicated substantially entirely from thermosetting resin and thegingival tissue was replicated substantially entirely from twothermoplastic resins. The model was formed using a rubbery dentalpoly(vinyl siloxane) impression material ("Express" Type 1, 3M) moldedupon a "Typodont" model ("R862", Columbia Dentoform) of two molar andtwo bicuspid teeth. One of the molar teeth had been prepared to receivea standard MOD restoration. Using a double-barrelled syringe equippedwith a static mixer ("EPX", 3M), Thermosetting Resin A was injected intothe impression to approximately the gingival margins. Over this,Thermoplastic Resin E was injected from a hot melt gun ("Polygun" TC,3M) at a melting chamber temperature of approximately 199° C. (390° F.).While Thermoplastic Resin E about 12 minutes with continued mixingfollowed by removal of the resin and casting into a slug mold suitablefor use in a hot melt gun. The slugs were identified as "ThermoplasticResin B".

Linear shrinkage of a bar of Thermoplastic Resin B (molded in the moldused for Thermosetting Resin (A) was 0.52%. The bar was cleanlybreakable when flexed by hand.

Thermoplastic Resin C

Using the same procedure used for Thermoplastic Resin B, the followingingredients were mixed with heating and stirring:

    ______________________________________                                        Ingredient              Parts                                                 ______________________________________                                        "Piccotex" 100          150                                                   Microcrystalline wax, m.p. 84-87° C.                                                            34                                                   ("Bowax" 993, Boler Chemical)                                                 "Imsil" A-25            200                                                   "Ti-Pure" R-960          10                                                   ______________________________________                                    

Linear shrinkage of a bar of the resulting Thermoplastic Resin C was0.043 cm (0.017 in.) or 0.34%.

Thermoplastic Resin D

Using the same procedure, the following ingredients were mixed withheating and stirring:

    ______________________________________                                        Ingredient                Parts                                               ______________________________________                                        "Piccotex" 100            370                                                 Polyethylene glycol dibenzoate                                                                           30                                                 ("Benzoflex" 2-45, Velsicol)                                                  "Imsil" A-25              250                                                 "Ti-Pure" R-960           200                                                 Carbon Black ("Sterling" R-V7688, Cabot)                                                                 3                                                  ______________________________________                                    

Linear shrinkage of a bar of the resulting Thermoplastic Resin D was0.005 cm (0.002 in.) or 0 04%.

    ______________________________________                                                                Parts                                                 ______________________________________                                        Part A:                                                                       Poly(glycidyl ether) of Bisphenol A                                                                     100                                                 having an epoxide equivalent weight of                                        about 200 ("Epon" 828, Shell)                                                 Silicon dioxide, mean particle size 4.3                                                                 20                                                  micrometers ("Imsil" A-25, Illinois Mineral)                                  Titanium dioxide, Sp. gravity 3.8-4.3                                                                   5                                                   ("Ti-Pure" R-960, E. I. duPont de Nemours)                                    Fluorocarbon surfactant ("Fluorad" FC-430,                                                              0.5                                                 3M)                                                                           Part B:                                                                       Polymercaptan ("Capcure" 3-800,                                                                         90                                                  Diamond Shamrock)                                                             Dimethylaminomethyl phenol ("DMP-30".,                                                                  10                                                  Rohm & Haas)                                                                  "Imsil" A-25              20                                                  "Ti-Pure" R-960           5                                                   "Fluorad" FC-430          0.5                                                 ______________________________________                                    

A slug of Thermosetting Resin A was cast into a bar 12.73×1.31×1.27 cm(5.012×0.516×0.500 inches). Linear shrinkage of the bar in the longdimension was 0.13%.

Thermoplastic Resin B

Using a hot plate (Corning PC-35) and a band heater (Tempco ElectricHeater Corp.), 50 parts of modified hydrocarbon-based resin, acid No.90-100 ("Pexalyn" A500, Hercules) were melted at a temperature of about171° C. (340° F.). While stirring with an air mixer, 150 parts of acopolymer of vinyltoluene and alpha-methylstyrene ("Piccotex" 100,Hercules) were added to the melt. When a homogeneous mixture had beenobtained, 225 parts of "Imsil" A-25 were added incrementally usinghigh-shear mixing, followed by incremental additions of 30 parts"Ti-Pure" R-960. The temperature was raised to 232° C. (450° F.) for wasstill molten, a serrated strip of tough and flexible thermoplastic resinwas pressed into the molten resin. The strip was about 100 mm in width,5 mm in average thickness, and 40 mm in length and was made by molding ahot melt adhesive having a Brookfield viscosity of 5,000 cps at 191° C.(375° F.), a tensile strength of 2.76 MPa (400 psi) and an elongation of750% ("Jet-Melt" 3792 hot melt adhesive, 3M) in a mold with a serratedface. After 10 minutes, Thermoplastic Resin E had hardened,Thermosetting Resin A had been cured by heat from the thermoplasticresin, and the resulting model could be removed from the dentalimpression. An excellent replica was obtained. Its replica tooth areaswere readily cut away with a bur. When a heated waxing spatula at atemperature of about 260° C. (500° F.)-was laid upon the replica teethfor several seconds, no damage to the teeth was noticed.

The model could be used to create dental restorations such as inlays byinjecting dental restorative or composite resin into the cavity of thereplica prepared molar. To facilitate doing so, the replica gingivaltissue formed by Thermoplastic Resin E was scored at the mesial anddistal sides of the replica prepared molar. Then upon flexing the modelby hand, the replica gingival tissue broke cleanly at each score,permitting the model to be hingedly opened in the manner shown in FIG. 2of the drawing. By grasping the replica prepared molar between thefingers, it and the replica gingival tissue between the cracks wereseparated from the serrated surface and lifted out to form a unitsimilar to unit 39 in FIG. 3, but having a replica molar formed entirelyof thermosetting resin.

EXAMPLE 2

A dental model was made as described in Example 1 except that instead ofinjecting a liquid thermosetting resin, a thermosetting epoxy resinpowder ("Scotchkote" 203, 3M) was sprinkled into the inverted dentalimpression from a plastic squeeze bottle. To facilitate application ofthe powder, the impression had been preheated in an oven at 65° C. Thedental impression was turned over, allowing excess powder to fall out,and leaving a uniform layer of powder covering the tooth surfaces andadjacent portions of the gingival surfaces.

The impression was once again inverted. Onto the layer of powder,Thermoplastic Resin E was injected from a hot melt gun as in Example 1to fill both the tooth and gingival portions of the impression. Aserrated strip of tough and flexible thermoplastic resin as used inExample 1 was pressed into the molten Thermoplastic Resin E. The modelwas allowed to cool, hardening within 10 minutes, at which time themodel could be removed from the dental impression and put to immediateuse to create a dental restoration. The thermosetting epoxy resinpowderhad been fused and cured by the heat of the molten Thermoplastic Resin Eto provide a thermoset shell having a uniform thickness of approximately1 mm. Surfaces of the model that had been covered by the thermoset shellwere readily cut away with a bur. When a heated waxing spatula touchedthe shell for several seconds, no damage to the teeth was noticed.

EXAMPLE 3

A dental model was made using a rubbery dental impression as inExample 1. Then using a double-barrelled syringe as in Example 1, atwo-part thermosetting urethane resin composition (-"Dyna-Cast",Kindt-Collins) was injected into the impression to approximately thegingival margins. Immediately thereafter, a tough and flexiblethermoplastic resin having a Brookfield viscosity of 14,500 cps at 191°C. (375° F.), a tensile strength of 3.3 MPa (475 psi), and an elongationof 600% ("Jet-Melt" 3758 adhesive, 3M) was injected from a hot melt guninto the impression to fill the gingival portion of the impression.After cooling for about 10 minutes, the resulting model was removed fromthe impression and was ready for immediate use in making dentalrestorations. The model was an excellent replica, its thermosettingresin having been cured by heat from the thermoplastic resin. Thereplica teeth of thermoset urethane resin could be readily cut away witha bur. When a heated waxing spatula touched the replica teeth forseveral seconds, no damage to the teeth was noticed.

The replica gingival tissue of the model was cut on either side of thereplica prepared molar to about half of the gingival tissue thickness of12 mm. The uncut replica gingival tissue then served as a hinge toprovide good access to the proximal surfaces of the replica tooth and toreturn the row of replica teeth approximately to the originalconfiguration. Even though the opposing surfaces at the cuts did notprecisely mate with one other, the resulting inaccuracy was deemed to beof only minor significance in the formation of typical restorations.

Various modifications and alterations of this invention will be apparentto those skilled in the art without departing from the scope and spiritof this invention and the latter should not be restricted to that setforth herein for illustrative purposes.

I claim:
 1. A hardenable model of an object useful for making arestoration for the object, said model having at least one surfaceportion of thermosetting resin which is not substantially thermoset,said model further comprising molten thermoplastic resin in contact withsaid thermosetting resin, said thermoplastic resin being cleanlybreakable at room temperature upon hardening.
 2. A model as defined inclaim 1, which replicates gingival tissue and a row of teeth.